Not Applicable
1. Field of the Invention
This invention broadly relates to analog and digital circuits requiring a reference voltage and more particularly relates to improvements in temperature dependent and temperature independent integrated circuits requiring a reference voltage.
2. Description of the Related Art
In analog integrated circuit (IC) designs there are temperature-dependent parameters in silicon devices such as bipolar transistors, field effect transistors (FET), diffusion resistors and poly-silicon resistors. Some circuit topologies are designed to cancel these temperature dependencies, but other circuit topologies have an inherent temperature dependence that is only canceled by a bias circuit. A bias circuit is controlled by a current or voltage source. These current or voltage sources are designed to have temperature dependence or temperature independence. There are applications where these sources are required to be accurate, and to have this accuracy, the bias circuit then requires either an external resistor or an internal trimmed resistor.
FIG. 1 is a prior art bandgap reference circuit (100) that generates an accurate bias current source which in turn is used to generate an accurate reference voltage, VBG. The accurate bias-current, Iacc., source is generated when using an accurate resistor, Rext., a resistor which is typically external to the IC. The following equation is for the accurate current source Iacc.       Equation   1:              I              acc        .              =                            V          BE                +                              2            ⁢                          R              2                        ⁢                          V              T                        ⁢                          ln              ⁡                              (                n                )                                                          R            1                                      R                  ext          .                    
The voltage, VBE, has a negative temperature coefficient while the thermal voltage, VT, has a positive temperature coefficient. Therefore, the ratio of R1 and R2 can be set so that the positive and negative coefficients cancel, thereby making the accurate current, Iacc., become temperature independent. On the other hand, the ratio of R1 and R2 can be set to favor either the positive or negative correlation, thereby making the accurate current become temperature dependent.
Since these two cases are mutually exclusive, an IC design ordinarily requires two external resistors: one external resistor for an accurate current source with temperature dependence; and a second external resistor for an accurate current source with temperature independence. In other words, all prior reference circuits can not generate an accurate current with temperature independence along with another accurate current source with temperature dependence. Because of this, all prior reference circuits require two precision external resistors (PERs); one for temperature independence and another for temperature dependence. Although the use of two PERs is useful, the use of two PERs does have its shortcomings, one short coming with the additional external component adds to the costs. Therefore, there is a need for an IC design that avoids the limitations of the prior art requirement of two external resistors to provide both temperature dependent and temperature independent circuits.
Another shortcoming with the use of two PERs is the resulting increase in physical size of the IC. IC designers strive to keep component count and component size to a minimum. Accordingly, a need exists for an IC design that overcomes the use of two PERs to provide both temperature dependent and temperature independent circuits. Accordingly, a need exists to eliminate the need for two external resistors and to provide a solution that uses only one PER to produce two accurate current sources: one accurate current source with temperature independence; and second accurate current source with temperature dependence.
Briefly, in accordance with the invention, disclosed is an accurate current source with an adjustable temperature dependence circuit. This type of accurate current source is used in silicon Integrated Circuit (IC) designs requiring supporting reference-voltage sources and or reference-current sources which may be designed with or without temperature dependence. The circuit generates an accurate current source with temperature independence along with another accurate current source with temperature dependence using only one precision external resistor. For the temperature-dependent current source, the temperature dependence can be controlled by setting a temperature dependence factor (TDF).